The present application relates to a battery using a positive electrode active material containing an alkali metal inclusive of lithium (Li) and an alkaline earth metal and to a positive electrode to be used for the battery.
Owing to the remarkable development of a portable electronic technology in recent years, electronic appliances such as mobile phones and laptop personal computers have started to be recognized as a basic technology supporting a high-level information society. Also, research and development on high functionalization of such an electronic appliance are energetically advanced, and the consumed electric power of such an electronic appliance increases steadily in proportion thereto. On the contrary, such an electronic appliance is to be driven over a long period of time, and realization of a high energy density of a secondary battery that is a drive power source has been inevitably desired. Also, in view of consideration of the environment, the prolongation of a cycle life has been desired.
From the viewpoints of occupied volume and mass of a battery to be built in an electronic appliance, it is desirable that the energy density of the battery is as high as possible. At present, in view of the fact that a lithium ion secondary battery has an excellent energy density, the lithium ion secondary battery is now built in almost all of appliances.
Usually, the lithium ion secondary battery uses lithium cobaltate for a positive electrode and a carbon material for a negative electrode, respectively and is used at an operating voltage in the range of from 4.2 V to 2.5 V. The fact that in a single cell, a terminal voltage can be increased to 4.2 V largely relies upon excellent electrochemical stability of a nonaqueous electrolyte material or a separator or the like.
For the purposes of realizing higher functionalization and enlarging applications on such a lithium ion secondary battery, a number of investigations are being advanced. As one of them, for example, it is studied to contrive to realize a high capacity of a lithium ion secondary battery by enhancing an energy density of a positive electrode active material inclusive of lithium cobaltate.
However, in the case of repeating charge and discharge at a high capacity, in particular, in a high-temperature region, a nonaqueous electrolyte coming into physical contact with a positive electrode is oxidatively decomposed, and a gas is generated to cause defectives such as blister, rupture, liquid leakage and the like of the battery. Also, a transition metal contained in the active material elutes into the nonaqueous electrolyte and is deposited on a negative electrode, thereby causing a fine internal short circuit; and therefore, there were caused such a problem that not only the safety is remarkably impaired, but deterioration of the capacity occurs, so that a battery life is shortened.
Then, a method in which a positive electrode active material is modified, thereby enhancing its chemical stability, suppressing elution of a transition metal into a nonaqueous electrolyte or the like and improving battery characteristics is investigated. Alternatively, a method in which a compound having a special function imparted thereto is added in a nonaqueous electrolyte, thereby forming a minute coating on either one of a positive electrode or a negative electrode or both of them and preventing deterioration of a battery capacity especially at a high temperature is widely adopted.
For example, Patent Document 1 (Japanese Patent No. 3172388) discloses a method in which a metal oxide is coated on the surface of a positive electrode, thereby improving a cycle characteristic. Also, Patent Document 2 (JP-A-2000-195517) discloses a method in which a metal oxide coating is formed on the surface of a positive electrode active material, thereby suppressing elution of a transition metal into a nonaqueous electrolyte and enhancing a battery life.
Patent Document 3 (JP-A-2002-270181) reports that when a phthalimide compound is incorporated into an electrode, and the compound having been dissolved in a nonaqueous electrolyte is adsorbed onto a positive electrode or a negative electrode, an effect for suppressing elution of a transition metal is obtained in the positive electrode, whereas deposition of the eluted metal is prevented in the negative electrode, whereby battery characteristics at a high temperature are improved. Also, Patent Document 4 (JP-A-2005-72003) reports that the addition of a nitrile derivative improves battery characteristics. At the same time, Patent Document 4 reports that in the case of using a mixed solvent such as a mixture of a cyclic or chain ester and a lactone, battery blister at the time of high-temperature storage can be suppressed.
In addition to the foregoing technologies, Patent Document 5 (Japanese Patent No. 3281701) reports that by adding calcium carbonate to a nonaqueous electrolytic solution, a free acid is removed in the electrolytic solution, so that battery characteristics are enhanced. Also, Patent Document 6 (Japanese Patent No. 3197763) reports that the addition of from 0.1% by mole to 20% by mole of a salt, hydroxide or carbonate of a transition metal such as manganese, cobalt and nickel, a typical metal such as aluminum and zinc, or sodium, magnesium or calcium to a positive electrode impairs a catalytic activity of a positive electrode material contributing to decomposition of an electrolytic solution, thereby enhancing storage characteristics of a battery.